Electric utility power distribution systems are frequently constructed underground for a variety of reasons ranging from objections to the above-ground aesthetics, the premium of above-ground space in dense urban locations, and safety concerns. Accordingly, power distribution systems heretofore constructed of poles, wires, and pole-mounted switches and transformers are being superseded and even replaced by underground systems in underground “vaults”.
Space in underground installations is at a premium, and material must be able to fit through municipal access holes, imposing strict dimensional restrictions on any such material. At the same time, environmental and safety concerns have discouraged the use of such dielectric materials as oil and SF6 which can be flammable and/or explosive while presenting environmental problems when leakage occurs or when emissions are created.
“Delta load” centers are located within underground vaults that are as much as a mile or more away from a utility substation. Customers receive power through these delta load centers. Each delta load center is comprised of three single-phase oil switch assemblies which each have four loadbreak switches connected to one another by a common bus. One loadbreak switch is connected to a feeder circuit and the other three are connected to radial branch underground circuits through paper-insulated lead cables (PILCs).
In order to provide power to the designated area, a three-phase feeder cable from the utility substation is brought to the delta load center, divided into three single cables which are each connected to the feeder loadbreak switch of an oil switch assembly. Three radial branch circuits are each connected to a loadbreak switch. Power is served to the customers when they are connected to the radial branch circuits.
The oil switch assemblies currently used in the delta load centers have typically comprised an electrically conductive bayonet-type switch element that is manually pushed in or pulled out between two electrically conductive terminals, one of which is connected to a common bus and the other is connected to the underground circuit. When inserted between the terminals, the bayonet electrically couples the terminals, completing the circuit and energizing the underground circuit. When manually pulled from the terminals, the switch breaks load current, “opens” the circuit, and de-energizes the underground circuit. The terminals and switch element are enclosed in a container that is oil filled.
The present invention pertains to current interrupter switches designed to replace oil switch assemblies used in underground “delta load” centers.